Add a separate error for dyn Trait in const fn

Previously "trait bounds other than `Sized` on const fn parameters are unstable"
error was used for both trait bounds (<T: Trait>) and trait objects (dyn Trait).
This was pretty confusing.

This patch adds a separeta error for trait objects: "trait objects in const fn
are unstable". The error for trait bounds is otherwise intact.

Author: WaffleLapkin

compiler/rustc_const_eval/src/transform/check_consts/check.rs

@@ -384,11 +384,11 @@ impl Checker<'mir, 'tcx> {
                         match pred.skip_binder() {
                             ty::ExistentialPredicate::AutoTrait(_)
                             | ty::ExistentialPredicate::Projection(_) => {
-                                self.check_op(ops::ty::TraitBound(kind))
+                                self.check_op(ops::ty::DynTrait(kind))
                             }
                             ty::ExistentialPredicate::Trait(trait_ref) => {
                                 if Some(trait_ref.def_id) != self.tcx.lang_items().sized_trait() {
-                                    self.check_op(ops::ty::TraitBound(kind))
+                                    self.check_op(ops::ty::DynTrait(kind))
                                 }
                             }
                         }

compiler/rustc_const_eval/src/transform/check_consts/ops.rs

@@ -599,7 +599,7 @@ pub mod ty {
         }
 
         fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
-            let mut builder = feature_err(
+            let mut err = feature_err(
                 &ccx.tcx.sess.parse_sess,
                 sym::const_fn_trait_bound,
                 span,
@@ -608,12 +608,51 @@ pub mod ty {
 
             match ccx.fn_sig() {
                 Some(fn_sig) if !fn_sig.span.contains(span) => {
-                    builder.span_label(fn_sig.span, "function declared as const here");
+                    err.span_label(fn_sig.span, "function declared as const here");
                 }
                 _ => {}
             }
 
-            builder
+            err
+        }
+    }
+
+    #[derive(Debug)]
+    pub struct DynTrait(pub mir::LocalKind);
+    impl NonConstOp for DynTrait {
+        fn importance(&self) -> DiagnosticImportance {
+            match self.0 {
+                mir::LocalKind::Var | mir::LocalKind::Temp => DiagnosticImportance::Secondary,
+                mir::LocalKind::ReturnPointer | mir::LocalKind::Arg => {
+                    DiagnosticImportance::Primary
+                }
+            }
+        }
+
+        fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
+            if ccx.const_kind() != hir::ConstContext::ConstFn {
+                Status::Allowed
+            } else {
+                Status::Unstable(sym::const_fn_trait_bound)
+            }
+        }
+
+        fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
+            let mut err = feature_err(
+                &ccx.tcx.sess.parse_sess,
+                sym::const_fn_trait_bound,
+                span,
+                "trait objects in const fn are unstable",
+            );
+
+            match ccx.fn_sig() {
+                Some(fn_sig) if !fn_sig.span.contains(span) => {
+                    err.span_label(fn_sig.span, "function declared as const here");
+                }
+                _ => {}
+            }
+
+            err
         }
     }
 

src/test/ui/consts/const_fn_trait_bound.rs

@@ -8,9 +8,9 @@
 const fn test1<T: std::ops::Add>() {}
 //[stock]~^ trait bounds
 const fn test2(_x: &dyn Send) {}
-//[stock]~^ trait bounds
+//[stock]~^ trait objects in const fn are unstable
 const fn test3() -> &'static dyn Send { loop {} }
-//[stock]~^ trait bounds
+//[stock]~^ trait objects in const fn are unstable
 
 
 #[rustc_error]

src/test/ui/consts/const_fn_trait_bound.stock.stderr

@@ -7,7 +7,7 @@ LL | const fn test1<T: std::ops::Add>() {}
    = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
    = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable
 
-error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
+error[E0658]: trait objects in const fn are unstable
   --> $DIR/const_fn_trait_bound.rs:10:16
    |
 LL | const fn test2(_x: &dyn Send) {}
@@ -16,7 +16,7 @@ LL | const fn test2(_x: &dyn Send) {}
    = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
    = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable
 
-error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
+error[E0658]: trait objects in const fn are unstable
   --> $DIR/const_fn_trait_bound.rs:12:21
    |
 LL | const fn test3() -> &'static dyn Send { loop {} }

src/test/ui/consts/min_const_fn/min_const_fn.rs

@@ -130,16 +130,16 @@ const fn no_apit(_x: impl std::fmt::Debug) {}
 //~^ ERROR trait bounds other than `Sized`
 //~| ERROR destructor
 const fn no_dyn_trait(_x: &dyn std::fmt::Debug) {}
-//~^ ERROR trait bounds other than `Sized`
+//~^ ERROR trait objects in const fn are unstable
 const fn no_dyn_trait_ret() -> &'static dyn std::fmt::Debug { &() }
-//~^ ERROR trait bounds other than `Sized`
+//~^ ERROR trait objects in const fn are unstable
 
 const fn no_unsafe() { unsafe {} }
 
 const fn really_no_traits_i_mean_it() { (&() as &dyn std::fmt::Debug, ()).1 }
-//~^ ERROR trait bounds other than `Sized`
-//~| ERROR trait bounds other than `Sized`
-//~| ERROR trait bounds other than `Sized`
+//~^ ERROR trait objects in const fn are unstable
+//~| ERROR trait objects in const fn are unstable
+//~| ERROR trait objects in const fn are unstable
 
 const fn no_fn_ptrs(_x: fn()) {}
 //~^ ERROR function pointer

src/test/ui/consts/min_const_fn/min_const_fn.stderr

@@ -279,7 +279,7 @@ LL | const fn no_apit(_x: impl std::fmt::Debug) {}
    |                  |
    |                  constant functions cannot evaluate destructors
 
-error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
+error[E0658]: trait objects in const fn are unstable
   --> $DIR/min_const_fn.rs:132:23
    |
 LL | const fn no_dyn_trait(_x: &dyn std::fmt::Debug) {}
@@ -288,7 +288,7 @@ LL | const fn no_dyn_trait(_x: &dyn std::fmt::Debug) {}
    = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
    = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable
 
-error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
+error[E0658]: trait objects in const fn are unstable
   --> $DIR/min_const_fn.rs:134:32
    |
 LL | const fn no_dyn_trait_ret() -> &'static dyn std::fmt::Debug { &() }
@@ -297,7 +297,7 @@ LL | const fn no_dyn_trait_ret() -> &'static dyn std::fmt::Debug { &() }
    = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
    = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable
 
-error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
+error[E0658]: trait objects in const fn are unstable
   --> $DIR/min_const_fn.rs:139:41
    |
 LL | const fn really_no_traits_i_mean_it() { (&() as &dyn std::fmt::Debug, ()).1 }
@@ -308,7 +308,7 @@ LL | const fn really_no_traits_i_mean_it() { (&() as &dyn std::fmt::Debug, ()).1
    = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
    = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable
 
-error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
+error[E0658]: trait objects in const fn are unstable
   --> $DIR/min_const_fn.rs:139:42
    |
 LL | const fn really_no_traits_i_mean_it() { (&() as &dyn std::fmt::Debug, ()).1 }
@@ -319,7 +319,7 @@ LL | const fn really_no_traits_i_mean_it() { (&() as &dyn std::fmt::Debug, ()).1
    = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
    = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable
 
-error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
+error[E0658]: trait objects in const fn are unstable
   --> $DIR/min_const_fn.rs:139:42
    |
 LL | const fn really_no_traits_i_mean_it() { (&() as &dyn std::fmt::Debug, ()).1 }

src/test/ui/consts/min_const_fn/min_const_fn_dyn.rs

@@ -7,9 +7,9 @@ struct Hide(HasDyn);
 const fn no_inner_dyn_trait(_x: Hide) {}
 const fn no_inner_dyn_trait2(x: Hide) {
     x.0.field;
-//~^ ERROR trait bounds other than `Sized`
+//~^ ERROR trait objects in const fn are unstable
 }
 const fn no_inner_dyn_trait_ret() -> Hide { Hide(HasDyn { field: &0 }) }
-//~^ ERROR trait bounds other than `Sized`
+//~^ ERROR trait objects in const fn are unstable
 
 fn main() {}

src/test/ui/consts/min_const_fn/min_const_fn_dyn.stderr

@@ -1,4 +1,4 @@
-error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
+error[E0658]: trait objects in const fn are unstable
   --> $DIR/min_const_fn_dyn.rs:9:5
    |
 LL | const fn no_inner_dyn_trait2(x: Hide) {
@@ -9,7 +9,7 @@ LL |     x.0.field;
    = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
    = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable
 
-error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
+error[E0658]: trait objects in const fn are unstable
   --> $DIR/min_const_fn_dyn.rs:12:66
    |
 LL | const fn no_inner_dyn_trait_ret() -> Hide { Hide(HasDyn { field: &0 }) }